WO2001076261A2

WO2001076261A2 - Automatic calibration of pan/tilt/zoom cameras

Info

Publication number: WO2001076261A2
Application number: PCT/EP2001/003556
Authority: WO
Inventors: Miroslav Trajkovic
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2000-04-04
Filing date: 2001-03-28
Publication date: 2001-10-11
Anticipated expiration: 2002-10-04
Also published as: JP2003530039A; TW505820B; EP1275240A2; WO2001076261A3

Abstract

The calibration of a pan/tilt/zoom camera uses a template image that is known to correspond to the optical center of a larger image. A first image is captured, at the camera's maximum zoom setting, that contains a scene containing distinguishing details. After capturing this image, the camera is maintained at the same orientation, set to its minimum zoom setting, and a second image is captured. The first image is scaled down to form a template image, using the known ratio of a camera's minimum and maximum zoom capabilities. The location of this template image within the second image defines the optical center, or principal point, of the camera's image plane. Having determined the optical center of the camera's image plane, the focal length of the camera at various zoom settings can be determined by measuring the displacement of a template image at the optical center corresponding to a slight adjustment of the camera through a known angle at each of the various zoom settings. Conventional curve fitting techniques are used for determining an algorithmic mapping between zoom settings and corresponding lens powers. The use of a lens power measure facilitates a linear determination of the coefficients of the mapping equation, and provides an objective function that is less sensitive to inaccurate focal length determinations at low zoom settings.

Description

Automatic Calibration of Pan/Tilt/Zoom Cameras

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of video systems, and in particular to the automation of cameras having pan, tilt, and zoom adjustments.

2. Description of Related Art

When a camera captures an image, the three-dimensional coordinates of objects within the field of view of the camera are mapped to two-dimensional coordinates of images of these objects. The mapping of object, or "world", coordinates to image coordinates can be used in a variety of applications to either control the field of view of the camera to include particular world coordinates, or to determine world coordinates based on image coordinates. For example, copending U.S. patent application "An Optimized User Interface for Selecting and Adjusting a Camera in a Multiple Camera System", Attorney docket 23,353, US serial number 09/002,105, filed 12/31/97 for S. Sengupta, D. Lyons, and T. Murphy, and incorporated by reference herein, discloses a user interface wherein the user selects a location on a displayed image, such as a floorplan, and in response to this selection, a controller adjusts one or more cameras so that their fields of view include the world coordinate corresponding to the selected location. Similarly, copending U.S. patent application "An Automated Camera Handoff System for Figure Tracking in a Multiple Camera System", Attorney docket PHA 23,489, US serial number 09/131 ,243, filed 8/7/98 for S. Sengupta, D. Lyons, T. Murphy, and D. Reese, and incorporated by reference herein, discloses a technique for automatically tracking a person or object as the person or object traverses from one camera's field of view to another by automatically controlling a camera based on world coordinates that are derived from images in another camera's field of view.

To provide an accurate model of the mapping between world coordinates and image coordinates that is performed by a camera, parameters must be provided that represent the relationship between the camera's settings and the captured image. The most often used model is a projective camera model and its mathematical form is: sm = Q(RP + T), (1) where m denotes the image coordinates of a world point P, s denotes the projective scale factor, Q is an internal camera calibration matrix, and R and T are the rotation and translation that align the camera and world coordinate systems. In order to use the camera for measurements between points in world and points in the image, it is necessary to determine the matrix Q. Determining the matrix Q is commonly termed "camera calibration". A common form of the matrix Q is:

where (xO, yO) denotes the optical center, or "principal point" on the camera's imaging plane, and f denotes the focal length. For ease of understanding, a common x and y focal length is shown; alternatively, the "f in the first row of the matrix Q can be expressed as "fx", the x focal length, and the "f" in the second row can be expressed as "fy", the y focal length. The optical center of a camera is the intersection of the camera's optical axis with the camera's imaging plane. The parameters of the matrix Q are typically expressed in pixels. A camera is typically calibrated by collecting data using varying camera settings, and then processing the data to determine, or estimate, the camera's optical center and focal length. If the camera has zoom capability, the process is repeated to determine the focal length corresponding to each of a variety of zoom settings. A calibration target is built by identifying the 3-D coordinates of reference points. The corresponding coordinates of these points in the camera image are determined for a variety of camera settings. Curve fitting techniques are employed to determine the parameters of equations (1) and (2), above, based on the mappings of the camera image coordinates and the actual 3-D coordinates of each reference point. U.S. patent 5,930,740, issued 27 July 1999, discloses a camera calibration method comprising establishing reference points within a 3-D space and triangulating the camera position and orientation with respect to the reference points. In the referenced patent, a database is created that maps the camera settings corresponding to each of a variety of camera positions and/or orientations.

To calibrate a zoom camera, it is necessary to determine the relationship between the control that is used to change the focal length and the effect produced by this change. For example, if a stepping motor is used to change the separation between lenses in a camera to increase or decrease the effective focal length, there is typically a non-linear relationship between a single step of the motor and the amount of change in the focal length. Calibration provides an algorithmic mapping to determine the camera's focal length at each step of the stepping motor, or other device that effects a change to the camera's focal length. For ease of reference, the term "zoom setting" is used herein as a generic term for a control of the camera that can be used to provide a repeatable zoom adjustment. That is, a "zoom setting of 7" will repeatedly provide substantially the same focal length, although this focal length may be unknown (before calibration). Conventionally, a camera's zoom setting is inversely proportional to the focal length.

A zoom setting of "8", for example, conventionally provides a focal length that is shorter (lower magnification) than a zoom factor of 7, a zoom setting of "6" provides a longer focal length, and so on. Typically, there are a large number of zoom settings available for a camera system, thereby facilitating a fine adjustment of focal length. Rather than determining and storing a field of view for each and every possible zoom setting, an algorithm is often determined to mathematically determine the focal length corresponding to each possible zoom setting. To determine the algorithmic mapping between a camera's zoom setting and corresponding focal length, a set of measurements z(i) and f(z(i)) are collected. For each of a plurality of zoom settings z(i), the corresponding focal length value f(z(i)) is determined, using for example the calibration techniques presented above. Because the focal length is inversely proportional to zoom settings (a high zoom setting corresponds to a short focal length), the conventional model used to determine a focal length corresponding to a zoom setting is an equation in the form:

f(z) = — ϊ • (3) l + aϊz + a2z² + a3z³ + ...

The order of the polynomial in the denominator of equation (3) is, in general, unknown. Experience indicates that an order of 2 is optimal for most camera zoom configurations. The coefficients aO, al, a2 that best match the set of measurements z(i), f(z(i)) for the camera are determined by minimizing the objective function:

C(a) = (4)

This direct determination of fitting the measurements of focal length, however, poses problems. As shown in equation (4), the objective function is nonlinear, and an iterative method is required to minimize the function. Also, the determination of focal length is often less reliable at low zoom settings (large focal lengths), because often there are fewer details (less information) in a zoomed-in image, and minor inaccuracies or variances in the camera settings or measurements will have a substantial effect on the determined focal length. A zoomed-out image, on the other hand, often contains more information than a zoomed-image image, and camera setting and measurement inaccuracies or variances have less effect on the determined focal length. That is, the larger values of f(z(i)) at low zoom settings z(i) are substantially less accurate than the lower values of f(z(i)) at high zoom settings z(i). As can be seen from the form of equation (4), the larger and more inaccurate measures of f(z(i)) will have a greater influence on the objective function than the smaller and more accurate measures of f(z(i)), and hence the determination of the coefficients aO, al, and a2 will be substantially affected by these inaccurate measures at low zoom settings.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a method and system for calibrating a pan/tilt/zoom camera that facilitates a more accurate calibration of the camera. It is a further object of this invention to provide a method and system for automatically calibrating a pan/tilt/zoom camera. It is a further object of this invention to provide a method and system for calibrating a pan tilt/zoom camera that is computationally simple.

These objects and others are achieved by providing a method and system for calibrating a pan/tilt/zoom camera that does not require the 3-D coordinates of objects to determine the mapping of a camera's settings to the optical characteristics of the camera. A first image is captured, at the camera's minimum zoom setting (zoomed-in as much as possible), that contains a scene containing distinguishing details. After capturing this image, the camera is maintained at the same orientation, set to its maximum zoom setting (zoomed- out as much as possible), and a second image is captured. The first image is scaled down to form a template image, using the known ratio of a camera's minimum and maximum focal lengths. The location of this template image within the second image is determined by searching the area in the vicinity of the center of the second image for a block that corresponds to the template image. Because the orientation of the camera is fixed between the first image and the second image, the location of this block defines the optical center, or principal point, of the camera's image plane. Having determined the optical center of the camera's image plane, the focal length of the camera at various zoom settings can be determined by measuring the displacement of the image at the optical center corresponding to a slight adjustment of the camera through a known angle at each of the various zoom settings. Conventional curve fitting techniques are used for determining an algorithmic mapping between zoom settings and corresponding field of view. In a preferred embodiment, the curve fitting is based on a determination of lens power rather than focal length. The use of a lens power measure facilitates a linear determination of the coefficients of the mapping equation, and provides an objective function that is less sensitive to inaccurate focal length determinations at low zoom settings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: FIG. 1 illustrates an example determination of the optical center of a camera system by using a template image in accordance with this invention.

FIGs. 2 A and 2B illustrate a technique for determining the focal length of a camera lens system based on a displacement of objects in an image in accordance with this invention. FIG. 3 illustrates the correspondence between the focal length of a camera and an apparent displacement of an object in an image as the camera is rotated.

FIGs. 4A and 4B illustrate an example flow diagram for a camera calibration process in accordance with this invention.

FIG. 5 illustrates an example camera system in accordance with this invention. Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an example sequence for the determination of the optical center (xO, yO) 101 of a camera system (FIG. 5) by using a template image 130 in accordance with this invention. Images 110 and 120 correspond to images captured by the camera system at two different zoom settings but at identical camera orientations (constant pan and tilt settings). Image 120 illustrates an image corresponding to a low zoom setting (long focal length) of the camera system. Preferably, this image 120 contains one or more clearly distinguishable objects, having a high degree of image-information, such as distinct edges and vertices. Image 110 illustrates an image corresponding to a higher zoom setting (shorter focal length) of the camera system, and corresponds to a "zoomed-out" image corresponding to the image 120. Preferably, the image 110 is captured at the minimum focal length, and image 120 is captured at the maximum focal length, so that the magnification ratio of the two images 110, 120 is known. That is, for example, a zoom lens is typically specified in terms of the ratio of focal lengths between maximum and minimum zoom. If the camera system has a "16x" zoom capability, and the images 110 and 120 are set to minimum and maximum focal lengths, respectively, the scale of image 120 will be 16 times the scale of image 110. Other techniques common in the art can alternatively be used to determine the scale of image 120 compared to image 110.

The template image 130 is created by scaling the magnified image 120 by the scale factor to produce the template image 130 at the same scale as the less magnified image 110. This template image 130 is then compared to the image 110 to determine the location of an area 140 within image 110 that matches the template image 130. Note that the search for a matching area 140 is preferably commenced at the image center of the image 110, and progresses outward from this image center until the matching area is found. In this manner, the likelihood of matching the template image 130 to a similar area at another location in the image 110 is minimal. Because the image 120 is preferably selected as an image having a high image-information content, the determination of a location of the template 130 within the image 110 can be very accurate, using pattern matching techniques that are common in the art. Because the orientation of the camera is identical between images 110 and 120, the location of the template 130 within the image 110 corresponds to the optical center of the camera system. Thus, as shown, a very accurate determination of the optical center of the camera system can be obtained by zooming into an arbitrary area within the camera's field of view that contains a distinguishable object image. Thereafter, the determination of the optical center can be automated, as discussed further below. As would be evident to one of ordinary skill in the art, panning for a distinguishable object in the camera's field of view can also be automated, thereby providing a completed automated camera calibration process. A template-finding approach to pattern matching is also used to determine the focal length of the camera system in a preferred embodiment of the invention. FIGs. 2A-2B and FIG. 3 illustrate a method of determining a camera's focal length in accordance with the principles of this invention. FIG. 2A illustrates a camera image 200. Within the camera image 200, a variety of object images 211, 212, etc. are displayed. FIG. 2B illustrates a second camera image 200' that is produced when the camera is tilted down slightly. The object images 211', 212', etc. in FIG. 2B correspond to the object images 211, 212 of FIG. 2A, and are illustrated as being displaced slightly, relative to their locations in FIG. 2A. FIG. 3 illustrates a lens 250, an image plane 260, and a focal point 270 that is located at one focal length f 255 from the lens 250. The optical axis of the lens 250 is illustrated by the line 251. Also illustrated in FIG. 3 is the lens 250 tilted down slightly, identified as lens 250', with a corresponding change in the optical axis, illustrated by the line 251', and corresponding change in focal point 270'. Assuming that the image plane 260 is fixed relative to the lens 250, and thus also rotates, the new location of the image plan 260 is identified at 260'. The distance D 265 corresponds to vertical displacement of the focal point. The displacement D 265 corresponds to an apparent displacement d 220 of the image on the image plane 260'. That is, the image 200' on the image plane 260' corresponds to an view that is slightly lower than the original view, and an object at the optical center of the image plane will appear to have been shifted by an amount d 220 compared to the original image 200. The distance D 265 and the focal length F 255 are in units of actual distances; for the purposes of image processing, these actual distances must be converted to image units. As can be seen, the displacement d 220 represents a conversion of the distance D into image units, and, based on the symmetry of the system, the image focal length f 225 represents a conversion of the physical focal length F 255 into image units. As can be seen, the tangent of the_. angle A is equal to the ratio of the distance D to the focal length E.

If the angle A that the lens is tilted is known, the image focal length f 225 can be determined as:

^f' ^ γ ⁽⁵⁾ where f is the focal length in image units, d is the displacement of the image on the image plane, in image units, and A is the angle that the lens is tilted to effect the displacement d. Referring to FIGs. 2A and 2B, assuming that the image plane corresponds to the displayed images 200 and 200', the displacement of the image can be determined by measuring the distance that each of the object images 211, 212, etc. are displaced between FIGs. 2A and 2B. The distances dl 221 and d2 223 are shown for clarification purposes. The displacement of the image can be determined as a composite of the individual object movement distances dl, d2, etc., such as an average, a least square error estimate, and so on.

The above focal length determination requires an automatic identification and location determination of corresponding points (211-211', 212-212', etc.) from one image to another, and is adversely affected by variances among determined distances dl, d2, etc. This variance can be significant if the camera image 200 contains object images that do not contain sharp edges, or contains similar object images in close proximity to each other. In a preferred embodiment, a template region 230 of an image 200 is used in lieu of the entire image, to reduce computation complexity, and to minimize the variances among determined distances between corresponding points of successive images. To find the focal length at a given zoom setting, a first image 200 is captured, and then the camera is tilted slightly by a known angle A and a new image 200' is captured. A template 230 is created by extracting a small image area from the first image 200, preferably centered about the above determined optical center 101 of the camera image 200. In this preferred embodiment, the amount of displacement that is introduced by the tilting through the angle A is based solely on the location of the template 230 in the new image 200'. The difference between the location of the template 230 in the new image 200' and the location of the template 230 in the original first image 200 corresponds to the displacement d 220 of FIG. 3, and equation (5), above.

Note that the selection of the template 230 as a small area at the optical center 101 facilitates an easy and accurate determination of the displacement d 220. The template 230 can be easily found in the new image 200' by searching along the displacement direction from the optical center 101' in the new image 200', and little, if any, ambiguity will be present if the selected template area 230 is substantially smaller than the entire image 200, 200'. Note also that the tilt-determined focal length f corresponds to a focal length in the vertical direction. If potentially different horizontal and vertical focal lengths, fx and fy, need to be determined, the template-matching process is repeated to determine fx via a slight panning movement of the camera system, in lieu of a tilt movement.

The conventional camera calibration process provides an algorithmic mapping between a zoom setting of the camera system and the corresponding focal length achieved at this zoom setting. To determine an algorithmic mapping of focal length to the zoom settings of the camera system in accordance with this invention, the above tilt-displacement template- matching technique is applied for a variety of zoom settings to collect sets of data for subsequent curve fitting to determine coefficients of a polynomial that models the relationship between zoom settings and focal lengths.

In accordance with this invention, a "lens power" term p is used to determine the algorithmic mapping between the zoom settings and the focal length, where p is the inverse off:

Because the lens power is the inverse of the focal length, and the focal length is inversely proportional to the zoom setting, the lens power is proportional to the zoom setting, and can be modeled as: p(z(i)) = b0 + blz(i) + b2z(i)² + ... (7) It has been found that an order of 2 is sufficient in this model for typical camera systems. By using the lens power in place of the focal length in this model, the objective function is expressed as:

C(a) = ∑(pz(i)) - (b0 + b\z(ι) + b2z(i)²)² . (8)

1=1 under constraint sp(z(i)_min ) - p(z(i)_maκ ) = 0 , which expresses the fact that the maximum zoom ratio is known. As can be seen, this objective function (8) is a linear function, for which a closed-form solution exists: b₀= ^q°^a ~ π _t (9) ^ai 22 ~ ^a\%⁽2\

b2=clb0+c2bl (11) where:

^el-«tfS max-« ""Wfrinin ^{,and (12)} _c2=^(_min-^)_{r; (13)}

Z( Vl / )max - SZi vl)mi ■n

n

«i2 = «2i = ∑ ^( (1 + c_xzιf )(1 + c₂z(i)) (15) ι=l

₂₂=∑z(0²(l + c₂z())² (16) ι=l

q₀=∑p(z(f))Q + cχϊ)²)² (17) ι^'=l q_l-∑z(i)²p(z(i))(l ₊ c₂z(i)) (18) ι=l where z(i)_mi_n is preferably the zoom setting at the minimum camera zoom setting, z(i)_max is the zoom setting at the maximum zoom setting, and s is the magnification factor that characterizes the ratio of the minimum and maximum zoom capabilities of the camera system. These equations assume that the ratio of the minimum and maximum zoom capabilities (the "magnification factor") of the camera is known, as is typical. If the magnification factor is not known, the coefficients can be determined using conventional curve fitting techniques.

As noted above, the accuracy of displacement measurements, and hence the accuracy of the power measurements, is higher at higher zoom settings. Also, at low zoom settings (high magnification), a very small tilt angle A effects a large displacement d 220. An error in either the adjustment of A or the measurement of d 220 will have a large effect on the determined focal length f, but a small effect on the determined lens power p. Thus, because the zoom settings and power measurements are proportional to each other, the more accurate higher valued power measurements will have a higher weighted affect on the determination of the coefficients bO, bl, and b2, and the more inaccurate lower valued power measurements will have a less weighted affect. FIGs. 4A and 4B illustrate example flow diagrams for a camera calibration process in accordance with this invention. FIG. 4 A illustrates a determination of the optical center, or optical center, of the camera, and FIG. 4B illustrates a determination of lens power as a function of the zoom settings of the camera.

At 410 in FIG. 4 A, the camera is oriented so that it contains a high amount of image-information at its maximum zoom setting, such as object images with clearly defined edges, and the image is captured as a reference image. This process is typically performed manually, although an automated panning and tilting process can be employed, such that the camera searches until it locates and captures a sufficiently discriminating image. At 420, a template is created by scaling the reference image down by the maximum zoom-ratio of the camera. That is, for example, if the camera is a "16X" zoom camera, the template is created by scaling the reference image down by a factor of 16.

The orientation used to capture the reference image is fixed while an image is captured 430 at the minimum zoom setting. Note that by maintaining a fixed orientation, the optical center the captured image will be common with the optical center of the reference image. At 440, image pattern matching techniques are used to determine the location of the template within the captured image at the minimum zoom setting, and this location defines the optical center of the camera.

The loop 450-464 of FIG. 4B, illustrates an example determination, or calibration, of the lens power corresponding to each of a plurality of zoom settings. The number of zoom settings selected for calibration to lens power is dependent upon the degree of precision required. Typically, between five and ten settings across the range of zoom settings are calibrated, to provide a set of five to ten points for a polynomial curve fitting of zoom settings to lens power, at 470. If the order of the polynomial is N, at least N+l calibration points must be provided. In a preferred embodiment, a polynomial of order 2 is used, and thus at least 3 calibration points must be provided.

At 452, the camera is oriented to an information laden center image, and the image is captured as a reference image at the current zoom setting. For convenience, the same orientation that was used to capture the reference image used to determine the optical center of the camera, at 410, can be used. At 454, a template is created by selecting a center portion of the reference image. Because the optical center of the camera is known, and the determination of the focal length, or its inverse, is based solely on the displacement of the optical center, as discussed above, the scaled template can be as small as a single pixel. In a preferred embodiment, the size of the template is inversely proportional to the zoom setting, and, for effective pattern matching, is at least 5x5 pixels in size.

At 456, the camera is offset by a known angle A, preferably in a vertical direction, to minimize the variables that may be introduced by a multi-dimensional displacement, and the displaced image is captured. At 460, the location of the template is determined within the displaced image. The distance between the optical center of the image and the displaced location of the template defines the image displacement D corresponding to the known angular displacement A. If the displacement of the camera is constrained to a vertical angle, and the optical center of the image is defined as an origin (0, 0), the displacement D is the magnitude of the vertical coordinate of the location of the template. The lens power, p, corresponding to the particular zoom setting is thereafter determined, at 462, using the inverse of equation (5) above:

P - f- - ^ . (14)

The lens power corresponding to each of the selected calibration zoom settings is similarly computed, via the loop 450-464. At 470, curve fitting equations (9)-(13) are applied to determine the best fit coefficients of a second order polynomial that maps zoom settings to lens power. Thereafter, given a zoom setting, the corresponding lens power is determined via equation (7) and the determined coefficients bO, bl, and b2, and the focal length is determined as the inverse of this lens power. FIG. 5 illustrates an example camera system 500 that is configured in accordance with this invention. The camera system 500 includes a camera 510, an image processing system 550, a controller 560, and a set of calibration parameters 570. The orientation of the camera is adjusted via a tilt motor 520 and pan motor 530, and the focal length of the camera is adjusted by the zoom motor and lenses 540. During the calibration phase, the controller 560 controls the motors 520, 530, 540 to execute the functions and sequences exemplified in FIGs. 4A-4B to derive the calibration parameters 570. The image processing system 550 performs a variety of functions, including: the capture of the images; the scaling of the reference image to create the scaled template that is used to determine the optical center; the determination of the location of each template in each image of the pair of images associated with each zoom setting; the corresponding determination of the lens power and/or focal length corresponding to each zoom setting; and the determination of the algorithmic mapping of the zoom setting to the lens power and/or focal length. After the calibration parameters are determined, the controller 560 receives requests for desired fields of view 580 and applies the calibration parameters and other factors to determine the appropriate commands to communicate to the motors 520, 530, 540, such as the appropriate zoom setting for the zoom motor 540, to achieve the desired field of view 580.

The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within the spirit and scope of the following claims.

Claims

CLAIMS:

1. A method of calibrating a camera (510) comprising: enabling a first adjustment of the camera (510) to a first zoom setting, enabling a capture of a first image (120) corresponding to a first camera view at the first zoom setting, enabling a second adjustment of the camera (510) to a second zoom setting, enabling a capture of a second image (110) corresponding to a second camera view at the second zoom setting, enabling a creation of a representative image (130) corresponding to the first image (120) scaled by a ratio of the first zoom setting and the second zoom setting, and enabling a determination of at least one calibration parameter based on a location of the representative image (130) within the second image (110).

2. The method of claim 1 , further including: enabling a third adjustment of the camera (510) to a tilt angle, enabling a capture of a third image (200') corresponding to a third camera view at the tilt angle, and enabling a determination of at least one other calibration parameter based on a location of the representative image (130) within the third image (200').

3. The method of claim 2, further including: enabling a fourth adjustment of the camera (510) to a pan angle, enabling a capture of a fourth image corresponding to a fourth camera view at the pan angle, and enabling a determination of at least one other calibration parameter based on a location of the representative image (130) within the fourth image.

4. The method of claim 2, further including: enabling a plurality of adjustments of the camera (510) to a plurality of other zoom settings, enabling a capture of a pair of images (200, 200') corresponding to each zoom setting of the plurality of other zoom settings, enabling a determination of additional calibration parameters based on a location of a template image (230) within each of the pair of images (200, 200') at each zoom setting.

5. The method of claim 4, wherein a difference (220) between the location of the template within the each of the pair of images (200, 200') provides an estimate of a focal length corresponding to each zoom setting, and the determination of the additional calibration parameters includes a polynomial curve fitting of the plurality of other zoom settings and the plurality of focal lengths.

6. The method of claim 5, wherein the polynomial curve fitting uses a polynomial model that is based on an inverse of the focal lengths.

7. The method of claim 1 , wherein the determination of the one or more calibration parameters includes a determination of the optical center (101) of the image plane (260) of the camera (510).

8. A camera control system (500) comprising: a zoom motor (540) that is configured to adjust a focal length of a camera

(510) based on a zoom value, a tilt motor (520) that is configured to adjust a line of sight of the camera (510) based on a tilt value, a pan motor (530) that is configured to further adjust the line of sight of the camera (510) based on a pan value, and a controller (560) that is configured to communicate the zoom value, the tilt value, and the pan value to adjust the camera (510) to a desired field of view based on a set of calibration parameters, wherein: the calibration parameters are based on at least one of: a location of a scaled image (130) of a first image (120) within a second image (110), and a determined distance (220) between locations of a template image (230) within each of a pair of camera images (200, 200') at each of a plurality of zoom values.

9. The camera control system (500) of claim 8, wherein the template image (230) in each pair of images (200, 200') is based on an area about the center of a first image (200) of the pair of images (200, 200'), the area about the center being substantially smaller than the first image (200).

10. The camera control system (500) of claim 9, wherein the area about the center is substantially proportional to an estimated focal length that is associated with each of the plurality of zoom settings.

11. The camera control system (500) of claim 8, wherein the determined distance (220) between the locations of the template image (230) within each pair of camera images (200, 200') provides an estimate of a corresponding focal length of a plurality of focal lengths corresponding to the plurality of zoom values, and the calibration parameters are based on a polynomial curve fit of the plurality of zoom values and the plurality of focal lengths.

12. The camera control system (500) of claim 8, further including: an image processing system (550) that is configured to determine the locations of the template image (230) in each pair of camera images (200, 200').

13. A method of calibrating a camera (510) comprising : enabling a determination of a plurality of lens power measurements, each lens power measurement of the plurality of lens power measurements corresponding to an each camera zoom setting of a plurality of camera zoom settings, and enabling a determination of curve fitting coefficients corresponding to the plurality of lens power measurements based on an objective function that gives a weight to each lens power measurement such that lens power measurements at higher camera zoom settings are given higher weights than lens power measurements at lower camera zoom settings.

14. The method of claim 13, wherein enabling the determination of the plurality of lens power measurements includes: enabling a determination of a first location of a template image (230) within a first field of view of the camera (510), and enabling determination of a second location of the template image (230) within a second field of view of the camera (510), and wherein the determination of each lens power measurement is dependent upon a difference (220) between the first and second location of the template image (230) within the first and second fields of view of the camera (510) at each camera zoom setting.

15. The method of claim 13 , further including: enabling a determination of an optical center of the camera (510) based on a location of a scaled image (130) within a camera image (110) obtained at a first camera zoom setting, wherein the scaled image (130) corresponds to a scaling of another camera image (120) at a second camera zoom setting that is higher than the first camera zoom setting.

16. A method of determining a focal length of a camera (510) comprising: capturing a first image (200) at a first orientation of the camera (510), offsetting the camera (510) by an offset angle A, thereby providing a second orientation of the camera (510), capturing a second image (200') at the second orientation, determining a displacement value d (220) corresponding to the offset angle A that is based on a displacement of one or more corresponding items (230) in the first (200) and second (200') images, and determining the focal length based on the displacement value d (220) and the offset angle A.

17. The method of claim 16, wherein the focal length f equals d/tan(A).